System and method for monitoring the performance of dense wavelength division multiplexing optical communications systems

ABSTRACT

A system and method for monitoring all the characteristic parameters of a DWDM communication system is implemented with two variants. Firstly, this is achieved by means of a specific grating spectrometer displaying a high resolution and a high-speed sampling of the measured values, and secondly by the application of an opto-electronic cross correlator as a purely electronic solution. The grating spectrometer is expediently a particular system in a mixed array according to Ebert and Fastie, wherein the light to be measured passes four times through the grating in a specific manner. The opto-electronic cross correlator can mix the working light with a reference light tunable in terms of its frequency to form an electrical low-frequency signal that is analyzed in a high-impedance operation.

RELATED APPLICATION

This application claims benefit of International Aplication No.PCT/EP99/097340, filed Oct. 5, 1990 having a priority date of Oct. 5,1998 based on DE 198 45 701.4. This application has not been publishedin English.

BACKGROUND OF THE INVENTION

The present invention relates to optical monitoring, and moreparticularly to a system and method of monitoring the performance ofdense wavelength division multiplexing optical communication services.

In densely packed WDM systems (dense WDM, DWDM) messages arecommunicated by light signals at different wave lengths via a singlefiber only. Each wave length is the carrier of an information signal.All channels are within the wave length range from presently roughly1,520 nm to 1,565 nm. The inter-channel separation amounts to a fewnanometers or some hundreds of picometers, respectively. Forstandardization of these telecommunication systems, the internationalITU-T Working Group has recommended the wave lengths (corresponding tothe channels) to be used with an inter-channel separation of 100 GHz(□0.8 nm) as standard. The ongoing development of these DWDM systemsaims at the extension of the utilizable wave length range up to 1,610 nmfor example.

Systems for the continuous monitoring of all characteristic parameterswith the possibility of signal regeneration or improvement are requiredat many sites of this communication system. The most importantparameters include the wave length and the capacity of all channels, themonitoring of the line width and the wave length drift of the lasers aswell as the signal-to-noise ratio in each communication channel. Typicalspecification requirements for monitoring are:

-   -   wave length measurement per channel with an absolute precision        of 0.08 nm and a resolution of 0.01 nm,    -   power metering per channel with an absolute precision of 0.4 dB        and a resolution of 0.1 dB,    -   S/N measurement between the channels with an absolute precision        of 0.4 dB at 0.1 dB,    -   reproducibility and a dynamic ratio of 33 dB at minimum,    -   reliability over 10¹⁰ measuring cycles (20 years approximately),    -   low PDL (0.1 dB max.),    -   small physical size.

Fundamentally different methods are suitable for monitoring purposes,which are employed in conventional optical spectrum analyzers.

Tunable narrow-band filters are used for wave length selection in thefiltering technique. Acousto-optical filters (e.g. those produced byWandel & Goltermann) or piezo-electrically controlled micro filters(e.g. those from the Queensgate company) or tunable fiber Bragg gratings(e.g. those from ElectroPhotonics Corp.) are applied, which can be tuneddirectly via an electrical parameter.

The filtering technique is not only restricted to the optical filteringoperation but it may also be performed at the electrical signal levelafter a preceding conversion into electronic signals. With electronicfiltering, the optical signal is mixed with an optical reference signalin a non-linear optical component while the differential frequencies areanalyzed on an electronic spectral analyzer (Hewlett Packard Co.).

Another variant is the grating monochromator technique wherein eitherthe grating is rotated or the spatially resolved signal spectrum issensed by means of a single photodiode, or the grating is stationary anda scanning deflection mirror is provided in front of the exit slit ofthe monochromator, or a mobile reflecting element varies the angle ofincidence of the radiation on the grating (e.g. Photonetics company), ora stationary grating is used in combination with a line of photodiodesas detector unit (e.g. Yokogawa company).

In the interferometric technique, the spectrum is obtained from thedetector signal of a Michelson interferometer with variable opticalpaths, with application of the Fourier transform (e.g. Hewlett Packardcompany).

None of the aforementioned conventional systems is suitable to satisfythe high demands made on a monitoring module for a DWDM system in termsof resolution, measuring accuracy, ASE measurement and dynamic ratio, atthe same time and in a suitable manner and to satisfy moreover thedemands in terms of short measuring intervals, longevity and low spacerequirements as well as low-cost realization.

What is desired is a suitable measuring system that satisfies thedemands on a DWDM monitoring system in terms of resolution, measuringaccuracy, ASE measurement and dynamic ratio, short measuring intervals,longevity and low space requirements as well as a low-cost production.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention this object is achieved with asystem permitting two variants. This aim is firstly reached inaccordance with the invention with a narrow-band tunable band-passfilter in the form of a specific grating spectrometer permitting a highresolution and a high-speed sampling of the measured values according toVariant 1, and secondly the solution according to the present inventionis presented in a Variant 2 as a purely electronic solution using anopto-electronic cross correlator.

The objects, advantages and other novel features of the presentinvention are apparent from the following detailed description when readin conjunction with the appended claims and attached drawing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows the fundamental structure of a narrow-band opticalband-pass filter having a grating spectrometer and an analyzer unit.

FIG. 2 illustrates the principle of an opto-electronic cross-correlator.

FIG. 3 shows the principle of a grating spectrometer with multiplepassages.

FIG. 4 illustrates an example of the structure and the optical path inthe grating spectrometer with multiple passages.

FIG. 5 shows the structure of a position sensor.

FIG. 6 illustrates an example of an opto-electronic cross-correlator.

FIG. 7 illustrates a beam combination by means of fiber couplers.

FIG. 8 is a view of a dual-channel opto-electronic cross-correlator.

DETAILED DESCRIPTION OF THE INVENTION Variant 1

FIG. 1 illustrates the fundamental structure of the embodiment includinga fiber input 5, a narrow-band tunable band-pass filter 1 and ananalyzer 3.

High-resolution spectrometers generally require several dispersive andimaging elements and are adjusted to the wave length to be detected in acomplex manner.

An example of a system based on a multiple spectrograph is illustratedin FIG. 3. The measuring light arrives through a fiber optical waveguide5 into the optical unit 13 including the spectrometer. The lightselected by a particular wave length arrives from the optical unit 13 onthe photo detector 11. The electrical signal obtained from the measuringlight in the photo detector is passed via a low-pass filter 6 to thesignal processor 7. There the wave length is assigned which thereference unit 9 has determined from the position signal 8 of theposition sensor 28 and which arrives at the signal processor 7, too.That processor generates also the necessary control signals for thedriving unit 10 and the grating drive 12 that adjusts thewavelength-determining element in the optical unit 13. Thecharacteristic values of the instantaneously set wave length, which arecalculated in the signal processor, are displayed to the user in thedisplay unit 14 and made available for being passed on.

The problem to achieve a high resolution is solved, in accordance withFIG. 4, by the structure of a specific grating spectrometer 1 wherein anechelle or ruled grating or a blazed grating for the wave length rangeto be monitored is mounted in a combined array according to Ebert andFastie and by approximation in a Littrow array. The optical paths forthe incident and exiting light are almost symmetrical there. Due to themultiple use of the grating and the single imaging element, that isequally provided for multiple use, in combination with several beamdeflection systems composed of flat mirrors or prisms, a compact,stable, highly dispersive and low-cost structure is achieved. Apredominantly symmetrical optical path in the optical unit reducesimaging errors resulting in a dramatic impairment of the resolution. Themovement of the grating for wave length selection can take place at ahigh speed because only a single element is moved. The application of asingle detector element only prevents site-dependent orelement-dependent variations in the responsiveness. Moreover, afurther-going independence from polarization effects such as PDL(polarization-dependent losses) is achieved because with the blazedgrating or the ruled grating, respectively, the beams are incident onthe diffracting grating surfaces almost orthogonally and cover a widegrating length at a high angle of incidence with a small beam diameter.

The angular position of the dispersing grating, that is decisive forassigning the measuring wave length, is determined by means of anauxiliary means, the position sensor, according to FIG. 5.

For a general grating the fundamental equationmλ=d(sin α+sin β)  (I)wherein m denotes the order, d represents the inter-line spacing and α,β indicate the angles of incidence or exit, respectively. As in aLittrow array grating the angles of incidence and exit are almostidentical, the definition according to Fastie furnishes the followingsimplified equation:mλ=2d sin α  (II)

In the definition according to Ebert the basic equation (I) applies. Theoptical path of the beams is so designed that the most symmetricaloptical path possible will be available with respect to the concavemirror. As in this case, too, the angles of incidence or exit are almostequal, the angular dispersion comes also under a similar magnitude orderas in the definition according to Fastie. Due to the multiplepassages—here quadruplicate, for instance—of the radiation through thedispersive element the overall dispersion and hence the resolution ofthe device is quadruplicated, too. On account of the utilization ofmirror areas n symmetrical positions, the symmetrical optical pathrelative to the imaging concave mirror results in an extensivecompensation of the imaging errors, particularly of astigmatism thatleads to a substantial deterioration of resolution.

With a dielectric optical preliminary filter as band-pass element in themultiple optical paths any light of wavelengths beyond the DWDM range issuppressed. In such a case the filter is then passed only by the DWDMrange, for instance, with a width of roughly 100 nm.

The detection of the entire spectrum is performed by a single radiationdetector while the adjustment of the wave length to be detected isrealized by rotating the grating about its vertical axis, which isperformed both by motor drive means and by the configuration asspring-mass array with torsion bars, capable of oscillating.

Furthermore, the position of the grating is detected by a secondarylaser with a very high precision. The focused beam of the secondarylaser is directed onto a reflecting surface rigidly connected to thegrating while the reflected beam is supplied to a position sensorincluding an incremental scale.

FIG. 4 illustrates an example of an appropriate embodiment. The light tobe examined arrives through the entrance opening, that is configured asfiber input 25, into the optical system. The diverging optical path isshaped by the collimator and camera mirror 27 to achieve a parallelpencil that is passed on by approximation onto the grating 24 at theblaze angle. The diffracted pencil then arrives again at the collimatorand camera mirror 27, is focused by the latter and arrives on themirrors 21 and 22 where it is deflected in a way that now the pencilwhich is divergent again, is passed along an axis parallel with the axisof the collimator and the camera mirror 27. The parallel pencil thenarrives at the grating 24 again, is diffracted there again and isincident on the collimator and camera mirror 27. From there, the beam isnow directed to the mirror 15 and via the mirrors 16, 17 and 18. Thebeam has now reached a position above the optical axis and is incidentagain on the collimator and camera mirror 27, arrives from there againat the grating 24 and arrives via the collimator and camera mirror 27 onthe grating 24 a second time. From there, the beam, that is nowdispersed even more strongly, arrives again at the collimator and cameramirror 27 and is passed from there to the mirrors 19 and 20, is incidenton the collimator and camera mirror 27 again, then on the grating 24,and then on the collimator and camera mirror 27 for the last time. Thefocused and four times dispersed beam then arrives at the signal output26. All beams arriving on the grating 24 several times and returned fromthere back to the collimator and camera mirror 27 again must passthrough the dielectric band-pass filter 23 and are cut there to theuseful frequency band.

FIG. 5 illustrates an example of a structure for detecting a position.The light of a secondary laser 41 is focused through the optical system42 on the incremental scale 45. The rotation of the grating 43 and theinvolved rotation of the mirror 44 rigidly connected to the gratingresults in a deflection of the laser beam over the incremental scale 45.

The influence which the incremental scale takes on the laser intensityis detected by the joining detector 46 and made accessible for analysis.

Variant 2

The Variant 2 according to FIG. 2—an entirely electronic solution in theform of an opto-electronic cross correlator 2—applies to methods knownper se from high-frequency technology. In this case, however, twooptical signals are mixed with each other without a previous conversioninto electrical signals. These two signals are firstly the working light5 to be examined and secondly the reference light originating from atunable laser 4. When the reference oscillator (laser) is tuned a beatfrequency is created whose frequency decreases as it approaches thefrequency of the working light; when the frequencies are equal itapproaches zero. This permits the use of components envisaged forapplication in the low-frequency range and hence also for the mixeroutput of a high-impedance load resistor. This results in a substantialimprovement of the responsiveness in detection. While the solutionsknown from the technique of optical superposition or interferenceoperate usually on a load resistance of 50 Ohm, this array allows forthe application of resistors of some kilo Ohm. The frequency range to beprocessed extends from a freely selectable lower frequency limit , thatis expediently higher than interfering mains frequency and base bandcomponents caused by the modulation of intensity of the opticalcarriers, up to an upper frequency limit which determines the bandwidthof integration. This frequency is expediently not substantially lowerthan the spectral width of the tunable laser acting as local oscillator.The advantage of such a system resides in the compact design, in theomission of mobile parts, in a purely electronic solution usingcomponents appropriate for application in the low-frequency range, inthe measuring rate restricted only by the tuning speed of the referenceoscillator, and in a high responsiveness at an almost optionally smallbandwidth of analysis.

The two light signals are defined by the following two relationships:E_(M) = E_(M)[i  ∫₀^(t)ω  t  𝕕t]  e_(M)E_(R) = E_(R)[i  ∫₀^(t)Ω  t  𝕕t]  e_(R)

This results in the following photo-electric current: $\begin{matrix}{I = {{E_{M} + E_{R}}}^{2}} \\{= {{E_{M}*E_{R}} + {E_{R}*E_{R}} + {2\quad{Re}\left\{ {E_{M}*E_{R}} \right\}}}} \\{= {E_{M}^{2} + E_{R}^{2} + {2\quad E_{M}E_{R}\quad{\cos\quad\left\lbrack {\int_{0}^{t}{\left( {\omega - \Omega} \right)\quad t\quad{\mathbb{d}t}}} \right\rbrack}}}}\end{matrix}$

It is apparent that the last term defines a current variable in time,that is dependent on the amplitudes of both radiations and on thedifference of the light frequencies. When both frequencies areapproaching each other a low-frequency signal is created with themaximum amplitude I_(max)=2 E_(M)E_(R). Moreover, the direction ofpolarization of both light sources is equally considered. In order toeliminate this dependence, it is possible, on the one hand, to renderthe reference light laser or the source of working light statisticallyvariable in terms of its direction of polarization, or, on the otherhand, to make two orthogonally polarized beams available, for instance,as reference light sources while the optical mixture is performed in twoseparate detectors with a subsequent logic operation in the signalprocessor. For another solution, for example, it is possible to switchthe reference laser over in a time-sequential manner in the polarizationplane while the subsequent measurements in succession are subjected to alogic operation in the signal processor.

FIG. 6 shows an example of Variant 2. The radiation to be measuredarrives as working beam via the fiber input 5 on a non-linear opticalcomponent, the detector 32. At the same time, the reference beam 40 ispassed via the polarizer 47 to the detector 32. The electrical mixedproducts generated from the optical signals arrive via the low-passfilter 33 to the rectifier 34 and from there at the digital signalprocessor 35 which realizes the evaluation of the signals, controls thedisplay unit 36 and supplies the reference laser controller 37 by meansof the tunable laser 38.

By employment of the wave length calibrator 29 for wave lengthassignment the provision of wave length references is made possible inboth variants. To this end known arrays such as absorption cells aresuitable for this purpose, which contain gases displaying characteristiclines of absorption in the required wave length range. When such a cellis inserted into the optical path, for instance in the spectrometer, andwhen the system is exposed to wide-band illumination characteristicsignal developments are created which permit a precise assignment of thewave lengths. Another possibility is the measurement o the referencelaser wave length by means of an additional interferometer array. Insuch a system, one part of the light from the tunable reference lightlaser is passed on to an interferometer that is provided with asupplementary highly precise light source and in which the interferencesignals vary in time, which are generated when the reference light laseris tuned, serve to assign the wave length present in that moment.

The combination of the working light and the reference light can berealized in different manners. FIG. 6 illustrates the free irradiationwith the measuring light, the reference light and possibly thecalibration light, which are incident on the non-linear detectorcomponent 32.

FIG. 7 shows that the various beams are combined by means of a fiberoptical component that is implemented in the form of a bulk or Y-typefiber coupler 48. The working signal at the fiber input 31 arrives viathe coupler 48 at the detector 32. The light of the reference laser 38is combined with the light of the wave length calibrator 29 via thepolarizer 47 in another coupler 48 and added to the working light in afirst coupler 48.

FIG. 8 illustrates an example of a dual-channel design permitting theconsideration of the aforementioned dependence on polarization. Theworking light is subdivided into two channels of orthogonal polarizationby means of a polarizing beam splitter 49. The reference laser 38 isequally split into two beams of orthogonal polarization and passed,together with the associated working beams, to two separate detectors46. The output signals of both detectors then arrive at the signalprocessor 35 where they are processed.

1. A system for monitoring the performance of DWDM multi-wavelengthsystems comprising: means for converting an optical signal for aparticular wavelength from the DWDM multi-wavelength system to anelectrical signal; means for processing the electrical signal todetermine the performance of the DWDM multi-wavelength system at theparticular wavelength and for controlling the converting means so thateach particular wavelength of the DWDM multi-wavelength system isprocessed and wherein the converting means comprises: means for mixingthe optical signal with a tunable reference optical signal to produce acombined optical signal; and a photodetector for converting the combinedsignal to the electrical, wherein the mixing means comprises: means fordividing the optical signal and the reference optical signal each intocorresponding orthogonal polarized beams; and means for combining therespective polarized beams of like polarization to form a pair ofcombined optical signals as the combined optical signal.
 2. The systemas recited in claim 1, wherein the photodetector comprises a pair ofphotodetectors having the respective combined polarization beams asinput and providing a pair of electrical signals at the respectiveoutputs as the electrical signal.
 3. A system for monitoring performanceof a DWDM multi-wavelength system comprising: means for converting aportion of an optical signal from the DWDM multi-wavelength system at aparticular wavelength to an electrical signal wherein the convertingmeans comprises an optical unit having the optical signal as an inputand the particular wavelength portion of the optical signal as an outputwherein the optical unit comprises a grating spectrometer having theoptical signal as an input and providing the particular wavelengthportion as an output wherein the grating spectrometer comprises amovable grating having a wavelength range that covers a measurementrange for the DWDM multi-wavelength system; an imaging element forreflecting the optical signal; and a beam deflection system mounted suchthat the optical signal incident on the imaging element and the opticalsignal exiting from the imaging element are essentially symmetrical, themovement of the movable grating selecting the particular wavelengthportion, and the optical signal being subjected to multiple passesbetween the movable grating and the image element; wherein the movablegrating is mounted with respect to the imaging element and the beamdeflection system in a combined array according to Ebert and Fastie andby approximation in a Littrow array; and means for processing theelectrical signal to determine the performance of the DWDMmulti-wavelength system at the particular wavelength and for controllingthe converting means so that each particular wavelength of the DWDMmulti-wavelength system is processed.
 4. The system as recited in claim3 wherein the converting means comprises a narrow-band tunable bandpassfilter having the optical signal as an input and the particularwavelength portion of the optical signal as an output.
 5. The system asrecited in claim 3 wherein the converting means comprises: aphotodetector having the particular wavelength portion as an input andthe electrical signal as an output.
 6. The system as recited in claim 5wherein the converting means further comprises a lowpass filter havingan input coupled to the output of the photodetector and having an outputto produce the electrical signal.
 7. The system as recited in claim 3wherein the grating spectrometer further comprises a dielectric opticalfilter situated between the movable grating and the imaging element sothat reflections of the optical signal between the movable grating andthe imaging element are bandpass filtered.
 8. The system as recited inclaim 3 wherein the movable grating comprises one selected from thegroup consisting of a ruled grating and a blazed grating.
 9. The systemas recited in claim 3 further comprising means for determining anangular position of the movable grating, the angular positiondetermining the particular wavelength portion.
 10. The system as recitedin claim 9 wherein the determining means comprises: a high precisionlight source for generating a focused beam; a reflecting surface rigidlycoupled to the movable grating upon which the focused beam impinges; anda position sensor for receiving the focused beam reflected from thereflecting surface to determine the angular position.
 11. The system asrecited in claim 9 further comprising means for moving the angularposition of the grating to select the particular wavelength portion. 12.The system as recited in claim 11 wherein the moving means comprises: adrive motor coupled to the movable grating for moving the movablegrating about a vertical axis in response to a control signal; aspring-mass array with torsion bars capable of oscillating coupled tothe drive motor; and means for driving the drive motor in response to acontrol signal from the controlling and processing means.
 13. The systemas recited in claim 10 wherein the position sensor comprises: anincremental scale that influences the intensity of the reflected focusedbeam as a function of the point on the incremental scale upon which thereflected focused beam impinges; and a detector for detecting anintensity of light from the incremental scale, the intensity being ameasure of the angular position.
 14. The system as recited in claim 3wherein the converting means comprises: means for mixing the opticalsignal with a tunable reference optical signal to produce a combinedoptical signal; and a photodetector for converting the combined opticalsignal to the electrical signal.
 15. A system for monitoring performanceof a DWDM multi-wavelength system comprising: means for converting aportion of an optical signal from the DWDM multi-wavelength system at aparticular wavelength to an electrical signal; wherein the convertingmeans comprises an optical unit having the optical signal as an inputand the particular wavelength portion of the optical signal as anoutput; and means for mixing the optical signal with a tunable referenceoptical signal to produce a combined optical signal wherein the mixingmeans comprises: a tunable laser for providing the tunable referenceoptical signal under control of the processing and controlling means;means for selectively polarizing the tunable reference optical signal toproduce a polarized reference optical signal in one of two orthogonallypolarized states; and means for combining the optical signal and thepolarized reference optical signal to produce the combined opticalsignal; a photodetector for converting the combined optical signal tothe electrical signal; and means for processing the electrical signal todetermine the performance of the DWDM multi-wavelength system at theparticular wavelength and for controlling the converting means so thateach particular wavelength of the DWDM multi-wavelength system isprocessed.
 16. The system as recited in claim 15 further comprising awavelength calibrator for providing a calibrated wavelength opticalsignal to irradiate the photodetector.
 17. The system as recited inclaim 15 wherein the combining means comprises simultaneous irradiationof the photodetector by the optical signal and the polarized referenceoptical signal.
 18. The system as recited in claim 16 wherein thecombining means further comprises simultaneous irradiation of thephotodetector with the calibrated wavelength optical signal as well. 19.The system as recited in claim 15 wherein the combining means comprisesa first optical coupler for combining the optical signal and thepolarized reference optical signal.
 20. The system as recited in claim19 wherein the combining means further comprises a second opticalcoupler for combining a calibrated wavelength optical signal with one ofthe optical signal and polarized reference optical signal prior tocombining with the other one in the first optical coupler.
 21. Thesystem as recited in claim 16 wherein the wavelength calibratorcomprises an absorption cell having a calibrated wavelength spectrum.22. The system as recited in claim 16 wherein the wavelength calibratorcomprises an interferometer array including a supplementary lightsource.